I just finished putting my first do it yourself computer (after nearly going in shock when my dad decided to each cruncy potato chips while I installed the CPU heatsink...). I've heard of people talking about "burning in" their computer for like 72 hours before doing anything. What does that mean? is there anything else I should do like this "burning in" before I start doing a lot of stuff?

For a long time it was considered good practice to "burn in" a computer when overclocking to improve stability. I have never found it nessisary to do this. Just go ahead and start playing with your computer. It is a good idea, however, (IMHO) to go and get a program like 3dMark or Sisoft Sandra and benchmark your system pretty heavily for a little while. This will help make sure that none of your new hardware has any hidden defects (like an IDE controller that corrupts data). Anyhow, good luck.